In conventional Diesel combustion processes, the requirements for low specific fuel consumption and for low toxic emissions oppose each other. However, operation of Diesel engines with two types of fuel makes it possible to fulfill both requirements at the same time. Another advantage of dual fuel operation consists in the fact that a considerable portion of the Diesel fuel can be replaced by alternative fuels, such as, for example, alcohols derived from vegetative materials, particularly ethanol.
Alcohols are distinguished by having extremely low cetane numbers. This signifies a low ignition propensity, so that these fuels cannot be burned in Diesel engines without taking further steps. The reason for this characteristic of alcohols and, particularly, of ethanol, is the substantially higher ignition temperature of alcohols compared to normal Diesel fuel. In addition, the heat of vaporization of ethanol is more than three times as high as that of Diesel fuel.
Various methods have already been used to increase the compression temperatures so as to prevent ignition failures when using ethanol in Diesel engines. Among these are: increasing the compression ratio, throttling and pre-heating the intake air, using exhaust gas recirculation and high temperature cooling. Investigations have shown that any single measure of the above cited kind does not suffice to guarantee partial load operation without ignition failures. Taking additional steps to aid in cold starting requires expensive controls and is too difficult for practical operation.
To avoid these disadvantages, it has already become known to add ethanol to the intake air through the carburetor or by low-pressure injection, i.e. to mix ethanol and air externally. Ignition then takes place after high-pressure injection of ordinary Diesel fuel. It was found that the percentage of ethanol could not be increased beyond 25% to 40% of the total fuel quantity, because knocking occured.
However, the percentage of ethanol could be increased substantially by two-fuel operation of the kind cited above, i.e. where alcohol and conventional Diesel fuel were injected into the combustion chamber of the cylinder by means of completely separate, high-pressure injection systems. However, this improvement was offset by the disadvantage of requiring two expensive high-pressure injection systems.
The German Federal Republic Offenlegungsschrift (Laid Open Patent Application) No. 15 76 478 describes a fuel injection valve, which makes possible a two-stage injection process of the same Diesel fuel into the combustion chamber of a cylinder. Such pre-injection tends to reduce the combustion noises of Diesel engines and provide relatively smoke-less combustion. The opening pressure for the main injection is greater than the opening pressure for the pre-injection, in order to obtain a distinct separation between the two injection processes.